Method and apparatus for determining distances of elastic discontinuities



March l0, 1942. R T, C| 0UD METHOD AND APPARATUS FOR DETERMININGDIsTANCEs 0F ELASTIC DISCONTINUITIES Filed June 23, 1939 AMPl/F/EARS'f/ME EAY MEA/VS /azg ,7 26 2/ 25 fo WA vf Gewezen rae fwrwuounyzer/Afa* fet-Quemar) INVENTOR Raymond 7." C/Oud r ATTORNEY body of waterat various points, etc.

Patented Mar. l0, 1942 UNiTEDg ."SATES i METHOD APPARATUS FOR DETERMIIL`ING DISTANCES F ELASTICDISCONTINU- ITIES Raymond T. Cloud, Tulsa, Okla.,assigner to Stanolind Oiland Gas Company, Tulsa, Okla.,

a corporation of Delaware Application June 23, 199, serial Noftjeoclaims. (Cl. 1x1-0.5)

back to a suitable point of reception.

My invention is adapted for use in a variety of operations, such as theacoustical determination of the altitude of an airplane or the likeabove the earth, the measurement .of the depth of a It is particularlyapplicable, however, to the more complex art of subsurfacerseismicprospecting, and will therefore be described in detail in thatconnection.

`In seismic/prospecting use is made of various refractions andreflections of elastic waves travcling through the formations of theearth. In `the. past, the best known method of seismic investigation hasincluded the steps of `creating a M sharp pulse of seismic energy nearthe surface of 4 the earth, usually by the detonation of aA charge ofexplosive, and receiving and recording the Yresultant waves impinging onseismometers at certain spaced points on the surface of the ground.

From a knowledge of the appearance of certain patterns of waves on therecord, it is possible to identify, with more or less dimculty, thewaves Which have beenreflected back to the surface from elasticdiscontinuities, and by determining in some way the average velocity ofthe particular waves involved, it is possible to compute the depths ofthe various reiiecting horizons.l

This method has a number of disadvantages. Since charges of explosiveare employed, there is always danger to some of the workmen and to theproperty on which the work is carried out. Second, intermittentdisturbances, such as the movements of animals, passage of trains,automobiles, etc., give rise to extraneous seismic waves v which are.recorded yand which confuse the interwhich of a large number ofrecorded waves are true reflections. Finally, the whole apparatus is icomplicated and cumbersome, and requires a large force of `men foroperation.

' I have devised an improved method and apparatus for seismicprospecting whereby these tages are obtained. Muy invention` utilizes acontinuous source of seismic waves, so that the hazards incident to theuse of explosives are'entirely absent. Another-feature of my inventionlies in the fact that no intermittent source of seismic energy can causea halt inexploration. Still another l'advantage of my invention overprior methods is thatl much less skill in operation and less equipmentare necessary, and. yet results are obtained in such a form thatinvolved study of records is eliminated. iv

It is therefore an object of myinvention to provide an improved methodof Vdetermining the distances of elastic discontinuities which is simplein operation and gives the desired informationrwith a minimum'ofuncertainty. Another object is toprovide a system of seismic prospectingwhich utilizes a continuous source of seismic waves in the mostadvantageous manner tondeltermine `the depth of reflecting undergroundstrata.fiA.further object is to provide a seismic v`sltufv'eying'"methodwhich is free from any hazard topersons orV property. Still anotherobject is "to provide relatively simple apparatus for steady 'stateseismic prospecting which is not sensitive to seismic waves ofextraneous origin. Further objects and advantages of my invention will.be apparent from rthe following description read in conjunction with thedrawing, in which:

Figure 1 shows a diagrammatic cross-section of anv idealized portion ofthe earths crust together with a schematic representation of onedisadvantages are eliminated and new advany form of apparatus accordingto my invention and utilized in carrying out a prospecting operation;and Figure 2 is similar to Figure l, but illustrates a modified form ofapparatus according to my invention.

In one of its broadest aspects my invention comprises the continuousgeneration of waves of varying frequency at one point in an elasticmedium suchas air, Water or-the earth, receiving Waves including thosereected from an elastic discontinuity at a second," associated point,converting the Waves receivcd into corresponding electrical variations,comparing these electrical variations by suitable indicating means witha set of electrical waves corresponding to the generated seismic waves,and modifying the electrical Waves in the set as to time and amplitudeuntil the modied waves are substantially the same as the electricalvariations. This procedure in effect results in the balancing out of thereected waves and any refracted and direct waves 4 dicated in Figure 1.

the drawing. In Figure 1 a diagram of a crosssection of the earth isshown in which a loose Weathered formation I0 extends below the surfaceto the topmost consolidated layer II. Below this are other strata, suchas formation I2 with an upper interface I3, and formation I4 `with anupper interface I5. For purposes of this illustration interfaces I3 andI5 .are-assumed to be capable of reflecting seismic waves. A source ofcontinuous seismic waves I6, which `is shown by way of example as avibrator, is set up at some point on the surface of the earth and isarranged so that the frequency of the generated waves is not constant,but varies as any arbitrary function of time. Essentially vibrator IE asshown in Figure 1 is made up of an unbalanced rotating v flywheel I1supported in a journaled support I8 which rests or is firmly attached tothe ground, the shaft of flywheel I1 being rotated. at a nonconstantrate by shaft I9 driven by motor 20, Alternatively vibrator i6 can bedriven by a nonconstant speed gasoline engine or cranked at an unevenrate by hand.

It is obvious that rotation of the unbalanced weight sets up a vibrationin the support I3 which in turn is transmitted to the ground, where itsets up seismic waves. These Waves propagate in all directions from thesource, and are refracted and reflected at elastic discontinuities inthe ground, as is well known to those working in this field. Fourvtypical wave paths have been in- One portion of the seismic energygenerated is transmitted along the surface of the ground, on a path suchas is shown by dotted line 2i.` A second portion traverses vthe longer'path shown by dotted line 22, being refracted due to the relativelygreat velocity of the Waves in formation Il as compared with weatheredlayer Id. A third portion follows the path 23 and is reflected frominterface I3, while a fourth portion follows path 24 and is reected frominterface I5. The reecting interfaces are of course normally much deeperthan the scale of the drawing would indicate.

Seismometer 25 is placed in contact with the ground at a considerabledistance, e. g."100 feet or more, from vibrator I6,.so that it will bevibrated by these waves. Preferably seismometer 25 is of theelectro-mechanical type, generating electric waves in response to thevibrations of the ground. These electric waves are conducted by cable 25to amplifier 21, which can be any of the well-known types and preferablyhas an. ad-

justable gain. It should introduce little or no distortion into theamplified signals.

A second, or reference seismometer 23 is placed relatively close tovibrator I6, so that the potential across its output terminals issubstantially in phase with the seismic waves generated therez by, andso that there is a fixed relation between these, and the amplitude ofthe electric signals produced by seismometer 28. These last signals areconducted by electric cable 29 to amplifier 30,

and 35 are connected to cable 3l in parallel sothat a set of electricalwaves eachof which corresponds to the seismic waves generated byvibrator I6, but lags by a desired period of time, is produced. Theoutput of each time-delay device is fed by cables 36, 31, 38 and 39,respectively, into an adjustable attenuating network such asfa T or Hpad or'faJ potentiometer so that the amplitude of each wave train can beindividually controlled. The attenuating network is illustrated simplyas a number of voltage dividers 40, 4I, 42 and 43 which receive theWaves in cables 36, 31, 38 and39, respectively, and deliver them atdesired reduced voltages in parallel to cable 44. The waves in cable 44therefore represent the sum of a number of waves, each ofcontrolledamplitude and each having a definite adjustable time lag withrespect to the seismic waves produced by vibrator I6.

The output of amplifier 21 and the combin output from the time-delay andattenuating devices are coupled 'in opposition to an. indicating device45, which is illustrated by way of example as a cathode-rayoscilloscope, in such a way that there is little or no effect of thesetwo sources of waves upon each other. This can, be done in a number ofways, for instance by supplying vthe output of amplifier 21 to theprimary coil of transformer 46 by means of cable 41, connecting cable 44across the primary'coil of transformer 48 and connecting the secondarycoils of trans-1 formers 46 and 48 in series but in opposition tooscilloscope 45 by conductor 49. Since the two electrical waves areconnected in opposition, the oscilloscope image will show a signal whichis the difference between the potentials across transformers 4E and 4Eat any given time.

In carrying out a prospecting operation using the apparatus of Figurelthe unbalanced flywheel I1 is rotated by motor 20 at a varying rate ofspeed, which may be arbitrarily chosen and does not need to be anyparticular function .of time. For convenience in operation it isadvantageous to have the variation in speed cyclical, but this is notnecessary. The rotation of ilywheel 2li imparts a varying force to theground upon which it is placed, thus generating seismic waves. Asmentioned "previously, one set of waves will be transmitted, roughly,along the surface of the ground as indicated by path 2|. The velocity ofpropagation of the seismic waves in this medium is relatively low,normally of the order of 1000 feet per second, so that there will be adefinite time delay between the time one particular wave was generatedby vibrator IG and the time that that same wave reaches seismometer 25.The wave traveling -along path 22 proceeds downward until it strikesthe'bed rock II at the base of the weathered layer IU, then travelsalong at the top of the bed rock and finally is refracted up through theweathered layer to strike seismometer 25. Usually the velocity ofseismic waves in this bed rock is very much higher than in the weatheredlayerl above, so that, although this path is longer than path elk-'aparticular wavilis'generated by vibratbrylGand the time that ally-less,or, inorefgen'erallly speaking', isAdi-fnthe' sarriejway it"cantbeshowfn -,tliat there vis a definite time'lag` associated :witheach `wave rerious elastic discontinuities along aths'ZS and24Jwhich,`in general, irorn'the time lags associatedwith pathsfffar'id22.` Iffvibratorv IB werefoperated at a constantspeed thewavessentoutnbyVV it' would be approximately sinusoidal `'and thefwav'esarrivingatl seismometer 25 would add up to give another"sinusoidaiwavewhich wouldV be out of phase with *the generated Wave due tothe timelags alongthewvari'ous paths. However, thisl -similar in shape at alltimes to'that sent out by vibrator IB. However, if the Wave sent out byvibrator 25 (which is picked up by seismometer 28 substantially asproduced). be divided into wave components and each component be delayeda time exactly the same as the Atime delay of one of the pathspreviously mentioned, and further, if each such component be attenuateduntil its amplitude is exactly a. the same as the amplitude of theparticularwave'train arriving at seisrnometer 25 along'the particularpath for which the time delay has been'adjusted, the resultant of thesecomponents will `be precisely the same in amplitude and will have thesame variation with time as the waves striking seismometer 25. Thus ifthese adjustments are made on the time delay devices 32, 33, 34 and 35,A

and attenuator circuits 40, 4I, d2 and 43, it is possible to balanceexactly theresultant wave from seismometer 25 acrosscoupling transformer46 with a similar wave' across transformer 48. In this case oscilloscope'45 will show no deection. If the Etime-delay and attenuating devicesare not so adjusted, it is impossible for the output from theattenuators to balance exactly the output from the sesmometer amplifier21 at all instants of time. Inasmucmach of the Waves through the earthis of varying frequency, it is impossible for it to add up with theother com;

` ponents in such a way that one will obtain the same Waves atseismometer 25 for two different locations in which the groundformations are arranged differently.

It will be seenfrom the above description that I balance the-response'of a seismometer resting on the earth and actuated by seismicwaves from a vibrator of.varying frequency against the response from adevice which corresponds to a dummy earth, so that after adjustment ofthe constants of the'^dummy earth until the oscilloscope 4-5 showssubstantially perfect balance, I can read off from the delay mechanismsthe actual travel-time of the various waves through the earthfand fromthe attenuator settings determine the relativeattenuation of thesewaves. By a knowledge of the average velocity of seismic it arrives atseismometer 25 along path 22? Wavesto the various reecting horizonswhich can be obtained in any of the methods well known to the art, suchas well shooting, I can compute from the various travel times the depthsto the various beds and from this knowledge can determine the dip of thevarious structures.

`It is not absolutely necessary to obtain a per-k tenuation along eachwave path is not obtained,

but this can be dispensed within many cases.

Though it is not always easy with the simple setup described todetermine which of the travel times are due to reflected waves, it isalways easy to determine this fact by `moving seismometer 25 a shortdistance from its original position, say 100 feet, and re-determiningthe various travel times. The travel times of waves .following therelatively direct paths 2| and 22 delay and attenuator devices directly,and

Various modifications of the above simple ern'-v bodiment can be madeWithout departing from the spirit of my invention, and some of these areshown in Figure 2, which is similar in operation to the apparatus ofFigure 1 and has many corresponding elements, which are given likereference characters. 2, in place of a reference seismometer, anelectric generator 6U is coupled to the shaft of motor 20 and theposition of the poles and coils is so adjusted that the electric outputthereof is exactly in tmephase with the seismic waves generated at thebase of vibrator I6. I can then operate upon this electric output withthe timeamplifier is necessary.

As stated above, I can use any type of time delay device, such. aselectrically long transmission lines, acoustical tubes or otherelectromechanical systems, which will produce a relativelydistortion-free time delay in a set4 ofV electrical signals. Aparticularly advantageous time-delay device is the magnetic discinstrument shown in Figure 2, which consists essentially of a metallicdisc 6I driven ata constant speed by motor 62 and shaft 63, and a numberof coils suitably arranged adjacent the outer edge of disc BI so thatthey all are in a magnetic relation with the same annular portion ofthedisc as it rotates.` The electric current pro` a manner known in theart, the magnetic varia-v tions corresponding to the inducing impulsesor variations. This magnetic path then cornes` into magnetic relationwith pick-up coils 66, 61, 68 and `69 in that order, so that electricalwaves are produced Ain each of these coils similar inA form to those incoil and therefore similar in form to the seismic Waves generated by vivrequired for a particular portion of the magnetic record to move thedistancefrom coil 65 Referring now to Figure IlOV the position of pickupcoil 66 is adjusted so that theA time lag of the waves induced thereinis equal to that of the seismic Waves reaching seismometer by refractedpath 22, that of coil 61 to give a time lag equal to that of direct path2|, and that of coils B8 and 69 to give time lags equal to those of thewaves following reflected paths 23 and 24, respectively, theseadjustments being made with the aid of oscilloscope 45 as previouslydescribedV in connection with Figure 1. The values of these time lagsare determined from the final positions of the respective pickup coilsand are used in the usual manner to determine the depth of the variousreflecting formations.

It is occasionally advantageous in balancing the dummy" earth to insertidentical adjustable low pass filters 12 and 13 before the transformers46 and 48. These lters are used in the early stages of preiiminarybalancing and as the amplitude of the waves on oscilloscope 45 becomessmall, are progressively cut out until they are finally uneifective.Numerous further modifications can be made in accordance with the spiritof my invention. For instance, I can incorporate an amplifier to amplifythe difference of the outputs of transformers `i4t and 48 beforeapplying this potential to the oscilloscope plates. The particular typeof coupling that is shown between the two eleccancel out the componentofthe emitted radio waves that travels directly from transmitter toreceiver.

While I have described my invention in connection with certain specificembodiments there.- of, I do not desire to be limited thereto, but onlyby the scope of the appended claims.

I claim:

1. The method of subsurface seismic surveying which comprises generatingcontinuous seismic waves of continuously varying frequency in the earth,receiving the resultant waves travelling at least two different Wavepaths at a point removed from the source of said seismic waves.transforming said received seismic wavesinto corresponding electricalvariations, producing electrical waves corresponding to said generatedseismic waves, and modifying said electrical `Waves with respect to timeand amplitude until said modified electrical waves are substantially thesame as said electrical variations.

2. The method of subsurface seismic surveying which comprises generatingcontinuous seismic waves in the earth, gradually and continuouslyvarying the frequency of said generated seismic waves, receiving seismicwaves reflected from at least one horizon and relatively direct seismicWaves resulting from said generated seismic waves, transforming saidreflected and relatively direct seismic waves into correspondingelectrical variations, producing a. set of electrical wavescorresponding to said generated seismic waves, balancing-.saidelectrical variations against said l set of electrical waves, andAadjusting the time lag trical wave trains and the cathode-rayoscilioscope need not be transformer coupling, but can be any of theother well-known types. Also other visual indicating devices can be usedinstead o1" a cathode-ray oscilloscope, for example a Vacuum tubevoltmeter can be advantageously employed. It is obvious that severalseismometers similar to seismometer 25 operating at varying distancesfrom vibrator i6 can be used, utilizveach of the seismometers in turn.

In applying this invention to the acoustical determination of the heightof an airplane above the earth, certain modifications would benecessary, such as the substitution of a source of acoustic Waves in airfor the vibrator I8, change of seismometers 25 and 28 to microphones,etc. These changes are, however, so obvious to one skilled in the artthat there is no need to describe them in any detail. The principle ofthe nvention remains the same regardless o1' the medium in which thegenerated waves propagate.

It is also possible to operate successfully under my invention asapplied to airplanes by using radio waves insteadv of sound waves. Thesewaves will'be reflected back fromthe earth to a` receiver, and theamplified received waves applied to the oscilloscope and balanced outwith a .time-delay and attenuator network as above. In this case muchshorter delay times will be found. Only one variable delay device needbe used since a fixed-delay device can be used to and amplitude of eachof the electrical waves in said set so that said electrical variationsand said set of electrical waves are substantially balanced out.

3. The method of subsurface seismic surveying which comprises generatingcontinuous seismic waves in the earth, continuously varying thefrequency of said generated seismic waves, receiving seismic wavesreflected from at least one horizon and relatively direct seismic wavesresulting from said generated seismic waves, transforming said reflectedand relatively direct vseismic waves into corresponding electricalvariations, producing a set of electrical waves corresponding to saidgenerated seismic waves, the number of electrical waves in said setcorresponding to the number of said reflected and relatively directseismic waves received, visually indicating a function of the differencebetween said electrical variations and said set of electrical waves, andadjusting the time lag and amplitude of each of the electrical waves insaid set until the indicated difference between said electricalvariations and said set of electrical waves is at a minimum, 1wherebythe depth of the reflecting horizons can be calculated from the time lagimposed on the corresponding electrical waves in said set.

4; The method of seismic surveying which comprises generating continuousseismic waves in the earth, the frequency of said generated seismicwaves being continuously varied, separately transforming seismic wavesinto corresponding electrical waves at two points at the surface of theearth, one of said points being adjacent, the point at which saidseismic waves are generated and the other of said points beingrelatively distant therefrom, dividing said eleccrical wavescorresponding to seismic Waves at said adjacent point into a pluralityof portions, introducing an adjustabletime lag and attenuation into eachof said portions, combining said portions to form electrical wavescorresponding to those which would be received at said relativelydistant point if the earth had a given structure.

coupling in opposition said last-mentioned waves and said electricalwaves, corresponding to the seismic waves actually received at saidrelatively distant point, observing the difference be tween said coupledwaves, and adjusting the time lag and attenuation of each of saidportions so that said diierence is reduced to a minimum.

5. Apparatus for seismic surveying comprising means for generatingcontinuous seismic Waves in the earth having a continuously varyingfrequency, means for receiving reflected, retracted, and relativelydirect seismic waves and transforming said waves intocorrespondingelectrical variatlons,means for producing electrical wavescorresponding to said generated seismic waves and means for modifyingsaid electrical waves with respect to time and amplitude until saidmodified electrical waves are substantially the same as said electricalvariations.

' `6. Apparatus for seismic surveying comprising means for generatingcontinuous sei/smic waves of continuously varying frequency inthe earth,at Aleast one seismometer lfor receiving seismic waves reflected fromunderground strata and Ysaid means for generating electrical wavescorresponding to said generated seismic waves comprises an electricgenerator actuated by said seismic wave generating means.

9. Apparatusv according to' claim 6 wherein said means for indicatingthe difference between s the waves in said set.

RAYMOND T. cLoUD.

